How to append with logic bricks?
To start with, I will just mention that the biggest issue in the current game market is not theft, it is the opposite: trying to get people to play your game out of the thousands of others.

My suggestion is to:


Provide a basic template to everyone (so who would steal it?)
Provide good documentation on what objects you can add that do what.


OR
Take a completely alternate approach, such as having a blend file called 'LevelParts.blend' with groups of objects people can append to their custom levels.